Indirect conversion nuclear battery using transparent scintillator material

ABSTRACT

A product includes a transparent scintillator material, a beta emitter material having an end-point energy of greater than 225 kiloelectron volts (keV), and a photovoltaic portion configured to convert light emitted by the scintillator material to electricity.

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The present invention relates to nuclear batteries, and moreparticularly, this invention relates nuclear batteries using transparentscintillator material and methods of making same.

BACKGROUND

Development of a long-lived high power density radioisotope battery hasrelied on incorporating either larger quantities of low energy emittingisotopes such as tritium (H-3) which limits the geometry of the batteryor smaller quantities of higher energy emitting isotopes such asstrontium-90 (Sr-90) and its daughter product yttrium-90 (Y-90). Someconventional radiovoltaic devices directly convert tritium beta decayenergy to electric current by using thin (˜200 nanometer thick)semiconductor layers but have limited power densities of less than 20μW/cm³. Other commercial devices employ higher energy beta emittingisotopes such as promethium-147 (Pm-147) but have only demonstratedpower densities of approximately 25 μW/cm³. Moreover, Pm-147 is scarce,does not occur naturally and requires complex chemical separationprocesses from spent nuclear fuel.

Some studies have shown that using alpha or high energy beta sources cangenerate greater power density compared to lower energy beta sources,e.g., beta sources such as tritium, Ni-63, etc., that generate muchlower power densities. For example, as shown in the schematic diagram ofa radiovoltaic battery 100 in FIG. 1A, using a direct conversionprocess, radiation energy (e.g., alpha (α) particles, beta (β) emissionparticles, etc.) may be converted to an electric current (I) through amulti-step process using semiconductor junctions (P-i-N). However, theseconventional radiovoltaic structures include semiconductor materials andhigh energy particles, e.g., alpha particles, high energy betaemissions, etc. tend to degrade semiconductor-based radiovoltaicsrapidly through defect production. Thus, conventional radiovoltaicstructures are restricted to being best suited for low power electricalapplications.

Recent contemplated approaches have suggested incorporating radiationhard scintillating material to convert radiation to photons. As shown inthe process as illustrated in the schematic drawing 120 in FIG. 1B, ascintillator 122 converts kinetic energy from ionizing radiation 124 tophotons 126 through excitation and emission. Early attempts to create abattery that functions to convert radiation to electrical energy havebeen unable to produce a viable result.

In some reports, combining single crystal inorganic scintillators,fluorescing powders, etc. with low energy beta emitting radioisotopeshas resulted in low power densities below 1 mW/cm³. For example, inconventional systems to generate light, lower energy beta emissionparticles (e.g. tritium, H-3) have been combined with scintillatormaterial (e.g., Exit signs) and demonstrate minimal degradation.However, in these products the low energy generated by tritium decayalso generates limited power densities. Moreover, in similarscintillator systems that replace the tritium with higher energyparticles such as x-rays, gamma particles, high energy beta emissionparticles, etc. the scintillator material used in these systemsdemonstrate significant degradation (e.g., point defects, etc.) duringirradiation with the higher energy particles.

Moreover, exposure to high energy beta emitting radioisotopes hasresulted in radiation-induced degradation and eventual failure ofscintillating material thereby limiting the duration of power generationand power density in these systems. For example, a device couplingscintillating material SrI and radioisotope Sr-90 suffered fromradiation-induced degradation and eventual failure of the SrI, therebyenabling only a very limited power density.

In addition, some scintillator materials may be damaged more rapidlyduring extended exposure to alpha particles compared to extendedexposure to high energy beta emission particles. For some scintillatormaterials, irradiation over extended periods of time results inincreased degradation of the material and progressively less lightproduction. For example, a single crystal inorganic scintillatormaterial, e.g., CsI, is bright, but is unstable in air, easily damagedby radiation, and sensitive to temperature.

It would be desirable to develop long-lived radioisotope batteriescapable of high power density for long periods of operation as anenabling technology to develop electrically powered systems in remotelocations or long-term applications.

SUMMARY

In one embodiment, a product includes a transparent scintillatormaterial, a beta emitter material having an end-point energy of greaterthan 225 kiloelectron volts (keV), and a photovoltaic portion configuredto convert light emitted by the scintillator material to electricity.

In another embodiment, a method includes forming a product having atransparent scintillator material, a beta emitter material having anenergy of greater than 225 kiloelectronvolts (keV), and a photovoltaicportion configured to convert light emitted by the scintillator materialto electricity.

Other aspects and implementations of the presently described inventiveconcepts will become apparent from the following detailed description,which, when taken in conjunction with the drawings, illustrate by way ofexample the principles of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic diagram of a conventional radiovoltaic battery.

FIG. 1B schematic drawing of the conversion of ionizing radiation to aphoton via a scintillator.

FIG. 2 is a schematic diagram of a product having a transparentscintillator material, beta emitter material, and a photovoltaicportion, according to one embodiment.

FIG. 3A is a schematic diagram of a photovoltaic device, according toone embodiment.

FIG. 3B is an image of a thin film photovoltaic device, according to oneembodiment.

FIG. 4A is a schematic diagram of a product having a transparentscintillator material, beta emitter material, and a photovoltaicportion, according to one embodiment.

FIG. 4B is a schematic diagram of a product having beta emitter materialintermixed with scintillator material, according to one embodiment.

FIG. 4C is a schematic diagram of a product having a homogenous mixtureof beta emitter material and scintillator material, according to oneembodiment.

FIGS. 4D-4E each depict a schematic diagram of a product having athree-dimensional structure such that the scintillator material has anon-planar surface that defines a space for deposition of beta emittermaterial, according to various embodiments.

FIG. 5A is a schematic diagram of a device including GYGAG(Ce)scintillator material, according to one contemplated approach.

FIGS. 5B-5C each depict a schematic diagram of a stacked diode design ofa device, according to some contemplated approaches.

FIG. 6 is a flow chart of a method, according to one embodiment.

FIG. 7A is an image of a GYGAG(Ce) transparent ceramic scintillator.

FIG. 7B is a plot of absorption and emission spectra of samples ofbeta-irradiated GYGAG(Ce) scintillator material.

FIG. 8A is a plot showing the radiation hardness of a GYGAG(Ce)scintillator in response to beta irradiation.

FIG. 8B is a plot showing the effect of alpha irradiation on two sidesof a GYGAG(Ce) scintillator.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

As also used herein, the term “about” denotes an interval of accuracythat ensures the technical effect of the feature in question. In variousapproaches, the ter “about” when combined with a value, refers to plusand minus 10% of the reference value. For example, a thickness of about10 nm refers to a thickness of 10 nm±1 nm, a temperature of about 50° C.refers to a temperature of 50° C.±5° C., etc.

For the purposes of this application, room temperature is defined as ina range of about 20° C. to about 25° C.

For the purposes of this application, radiation resistance is measuredin terms of resistance to an absorbed dose in units of rad, such thatone rad is equivalent to 100 erg/g or 0.01 Gray (Gy). One megarad (Mrad)is equivalent to one million rads. One gigarad (Grad) is equivalent to1000 Mrads.

The following description defines a material being transparent asallowing light to pass through so that objects behind may be distinctlyseen. Transparency of a material is defined as having a luminoustransmittance value of at least 85% of a given wavelength or range ofwavelengths of light passing through the material. This corresponds tooptical scatter of typically less than 20% per centimeter (cm) ofmaterial. Optical scatter is measured via scatterometry at thescintillation wavelength which involves passing a laser beam through asample held in an integrating sphere; and the resulting scatteredphotons into the sphere are detected as optical scatter. In some cases,a degree of transparency may also be defined as not opaque, opticallyclear, etc. These are by way of example only and are not meant to belimiting in any way.

Unless expressly defined otherwise herein, each component listed in aparticular approach may be present in an effective amount. An effectiveamount of a component means that enough of the component is present toresult in a discernable change in a target characteristic of the finalproduct in which the component is present, and preferably results in achange of the characteristic to within a desired range. One skilled inthe art, now armed with the teachings herein, would be able to readilydetermine an effective amount of a particular component without havingto resort to undue experimentation.

The description herein is presented to enable any person skilled in theart to make and use the invention and is provided in the context ofparticular applications of the invention and their requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art upon reading the present disclosure, includingcombining features from various embodiment to create additional and/oralternative embodiments thereof.

Moreover, the general principles defined herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the present invention. Thus, the present invention is not intended tobe limited to the embodiments shown but is to be accorded the widestscope consistent with the principles and features disclosed herein.

The following description discloses several preferred inventive conceptsof an indirect conversion nuclear battery using transparent scintillatormaterial and/or related systems and methods.

In one general embodiment, a product includes a transparent scintillatormaterial, a beta emitter material having an end-point energy of greaterthan 225 kiloelectron volts (keV), and a photovoltaic portion configuredto convert light emitted by the scintillator material to electricity.

In another general embodiment, a method includes forming a producthaving a transparent scintillator material, a beta emitter materialhaving an energy of greater than 225 kiloelectronvolts (keV), and aphotovoltaic portion configured to convert light emitted by thescintillator material to electricity.

A list of abbreviations and acronyms used in the description is providedbelow.

2D two-dimensional

3D three-dimensional

C Celsius

CsI cesium iodide

cm centimeter

Grad gigarad

GYGAG gadolinium yttrium gallium aluminum garnet

H-3 tritium

InGaP Indium gallium phosphide

keV kiloelectron volts

Kr-85 krypton-85

LuAG lutetium aluminum garnet

LYSO lutetium-yttrium oxyorthosilicate

MeV megaelectron volt

μm micron

mW milliwatt

μW microwatt

mm millimeter

Mrad megarad

nm nanometer

Pm-147 promethium-147

RT room temperature

Si silicon

Sr-90 strontium-90

SrI strontium iodide

Tl-204 thallium-204

Y-90 yttrium-90

Various embodiments described herein operate using the principle ofconverting the kinetic energy from a radioisotope decay particle via anindirect step into photons and then converting those photons intoelectricity. As described herein, a device includes a radiation sourceproviding ionizing energy that enters a scintillator material, the layerof the scintillator material in turn converts the ionizing energy tophotons comprising light energy, and an adjacent photovoltaic portioncoupled to the scintillator material in turn converts the light energycarried by the photons to electrical energy carried by electrons, e.g.,a current. In one approach, the device as described herein is a battery.

According to one embodiment, a method includes constructing a long-lifeelectrical power generation device powered by the decay of aradioisotope using a radiation-hard scintillator and a photovoltaicportion. In one approach, the device includes a transparentpolycrystalline ceramic garnet scintillator activated by a rare earthelement in physical contact with a long-lived beta emitter, e.g., suchas thallium-204 (Tl-204), Sr-90, etc., to generate photons. Thesystem-generated photons are converted to electricity through aphotoelectric effect by way of photovoltaics adjacent the scintillatorand radioisotope in a layered structure.

As described herein, according to various embodiments, a radiovoltaicdevice includes scintillating material that tends to be more radiationhard (resistant to degradation due to exposure to ionizing radiation)than semiconducting materials, and that efficiently converts damagingradiation into photons. In one approach, a radiovoltaic devicepreferably maximizes the optical transport of a beta emission withhighest selection efficiency.

According to one embodiment, a product may be a two-dimensional (2D)structure, for example, a planar structure. The product includes atransparent scintillator material, a beta emitter material having anend-point energy of greater than 225 kiloelectron volts (keV), and aphotovoltaic portion configured to convert light emitted by thescintillator material to electricity. End-point energy is defined as themaximum energy which can be imparted to the beta particle as a result ofthe kinematics of the 3 body system of beta decay for a givenradioisotope decay energy. A transparent material is defined as having atransparency and dimensions such that a luminous transmittance value ofat least 85% of the light created within the scintillator materialpasses out of the scintillator material.

FIG. 2 depicts a product 200, in accordance with one embodiment. As anoption, the present product 200 may be implemented in conjunction withfeatures from any other embodiment listed herein, such as thosedescribed with reference to the other FIGS. Of course, however, suchproduct 200 and others presented herein may be used in variousapplications and/or in permutations which may or may not be specificallydescribed in the illustrative embodiments listed herein. Further, theproduct 200 presented herein may be used in any desired environment.

As illustrated in FIG. 2, a product 200 has a single stack 206 of layersthat include a layer 207 of transparent scintillator material 202 (e.g.,a light generation cell), a layer 205 of beta emitter material 204, anda photovoltaic portion 214 configured to convert light emitted by thescintillator material 202 to electricity e⁻. The photovoltaic portionmay be a single power producing unit.

A beta (β) emission 201 from the beta emitter material 204 passes intothe scintillator material 202 where the beta emission 201 is convertedto a photon 209. The photovoltaic portion 214 is configured to convertthe photon 209 emitted by the scintillator material 202 to electricitye−, e.g., electrical current. as illustrated in the simple schematicdrawing of a magnified view 218 of a photovoltaic effect.

In addition, the stack 206 of layers may include a reflective layer 203on the upper surface of the layer 205 of beta emitter material 204. Insome approaches, a reflective layer may be included on at least one ofthe sides of the layer of scintillator material in a z-direction of thethickness of the layer in order to prevent light from escaping from thesides of the layer of scintillator material.

As a radiation source, the beta emitter material 204 may include highenergy beta emission particles. In various approaches the beta emittermaterial has an energy above a level that typically damages mostsemiconductor materials. In some approaches, the beta emitter materialhas an energy (e.g., radiation energy) of greater than 100 keV. Inpreferred approaches, the beta emitter material has an energy of greaterthan 225 keV. In some approaches, the beta emitter material may have anenergy less than 200 keV.

In some approaches, the beta emitter material is preferably metallichaving a maximum density. In one approach, the beta emitter material mayinclude thallium-204 (Tl-204, Tl metal), strontium-90 (Sr-90),krypton-85 (Kr-85), argon-39 (Ar-39), Ar-42, yttrium-90 (Y-90),cadmium-113m (Cd-113m), etc. In a preferred approach, Tl-204 having amaximum radiation of 750 keV may be an optimal beta emitter material. Insome approaches, beta emitter material may have an average radiationenergy of less than about 1 MeV, for example, strontium-90 (Sr-90). Insome approaches, the beta emitter material may include a combination ofbeta emitter materials, e.g., Tl-204 and Sr-90. Tl-204 and Pm-147, etc.

In various approaches, the amount of beta emitter material may beinversely coordinated to the desired total power density of the layer ofbeta emitter material. For example, a thicker layer of beta emittermaterial promotes increased self-absorption, thereby in turn decreasingefficiency of extracting energy of the decay for transforming intophotons. In some approaches having volume constrained designs in whichthe radioisotope material may be limited to a maximum surface area, thethickness of the layer of radioisotope material may allow higher amountsof radioisotope thereby possibly decreasing efficiency without affectinglight output. The efficiency of collected energy from the beta emittermay decrease with thicker layers, but the total energy collected tendsnot to decrease, and thus light output may be unaffected. Thus, in someapproaches, as the amount of beta emitter material increases, lightoutput production efficiency may decrease.

According to one embodiment, a system incorporates recently developedpolycrystalline ceramic garnet scintillators having improved radiationhardness. The brightness of the polycrystalline ceramic garnetscintillators is preferably comparable to that of single crystalinorganic scintillators.

The scintillator material 202 is preferably a radiation hardscintillator material. The radiation hard scintillator material may becharacterized as not exhibiting a significant (greater than 10%)degradation of light output under continuous exposure to radiationenergy at 1 megaelectron volt (MeV) to a dose of at least 1 gigarad(Grad) over the duration of one year. In one approach, the continuousexposure to radiation may include exposure to 1 MeV to a cumulative doseof greater than 1 gigarad over a duration of one year. In one approach,the scintillator material may exhibit a light output of greater than30,000 photons per megaelectron volt (MeV).

In one approach, the scintillator material may be in the form of aceramic. In preferred approaches, the scintillator material is in theform of a transparent ceramic. In exemplary approaches, the scintillatormaterial includes a transparent ceramic. In preferred approaches, aceramic scintillator material has improved radiation hardness oversingle crystal scintillator material. In another approach, thescintillator material may be in the form of a single crystal material.The single crystal material may be formed from sintered particles, meltgrown, ball milling of the materials.

In one approach, the scintillator material may be in the form of apowder. Illustrative examples of powders include ZnS:Cu, ZnS:Ag, etc.

In some approaches, the scintillator material 202 may include aninorganic scintillator material. In a preferred approach, the inorganicscintillator material is polycrystalline ceramic, e.g., lutetiumaluminum garnet (LuAG), lutetium-yttrium oxyorthosilicate (LYSO),gadolinium yttrium gallium aluminum garnet (GYGAG), etc. Polycrystallineceramics tend to be robust, radiation-hard, and low-cost. In particular,gadolinium yttrium gallium aluminum garnet (GYGAG) scintillator materialoffers several desirable properties: an emission spectrum centered at2.2 eV, well matched to photovoltaic conversion, material that ismachinable, a solid-state material, etc.

In one approach, at least some of the scintillator material includes anactivator such as cerium (Ce), europium (Eu), samarium II (SmII), copper(Cu), silver (Ag), thallium (Tl), etc. Activators may be used as dopantsof the scintillator material in an effective amount for promoting highlight yields. For example, in an exemplary approach, the scintillatormaterial includes a cerium-activated gadolinium yttrium gallium aluminumgarnet (GYGAG(Ce)). A scintillator material doped with cerium mayprovide faster luminescence thereby preventing a dissipation of energyto undesirable reactions. A faster decay time may promote a moreradiation hard material. In preferred approaches, rare earth-basedscintillators activated with cerium in particular demonstrate improvedradiation hardness over alternative scintillators.

Cesium-doped GYGAG, (GYGAG(Ce)) ceramic scintillator is a bright,radiation-hard scintillator. In one example, GYGAG(Ce) is a transparentceramic scintillator. A typical formula for GYGAG(Ce) isGd_(1.5)Y_(1.5)Ga_(2.2)Al_(2.8)O₁₂(Ce). The density of a GYGAG(Ce)scintillator material is 5.8 g/cm³. In one approach, the incidentradiation-to-photon conversion efficiency of a GYGAG(Ce) material for anemission photon exhibiting about 2.2 eV may be in a range of about 8 to11% (for the spectral range of 450 nm to 700 nm, as shown in FIG. 7B,Experiments section).

In one approach, the scintillator material demonstrates radiationhardness in response to beta irradiation. In recent approaches, aradiation hardness of the material may be on the order of 10s to 100s ofgigarad (Grad). In one exemplary approach, a device having a transparentceramic GYGAG(Ce) scintillator material shows no degradation for thefull lifetime of the device. For example, irradiation of a transparentceramic GYGAG(Ce) scintillator material with 1 MeV electrons to afluence of 1e17/cm² and dose of approximately 2.9 Grad and with 750 keVelectrons to a fluence of 1e19/cm² and a dose of up to 310 Graddemonstrates essentially no degradation of radioluminescence. Moreover,essentially no difference in scintillation efficiency may be determinedbetween two opposite sides of a scintillator sample, followingirradiation with 1 MeV electrons that do not penetrate the fullthickness of the scintillator (as shown in FIG. 8A, Experimentssection). Without wishing to be bound by any theory, it is believed thata higher irradiation dose may allow for more accurate extrapolation toan expected battery outcome.

In some approaches, alpha particle irradiation of GYGAG(Ce) scintillatormaterial may not be efficient for power generation. Using a fraction ofthe number of particles as may be used in beta irradiation of a similarsample, the emission maxima may demonstrate greater than 90% degradationof light yield compared to the non-irradiated side of the scintillatorsample (as shown in FIG. 8B, Experiments section).

In one approach, the scintillator material may include single crystalgrowth material. However, in some instances, comparing the GYGAG(Ce)scintillator to a single crystal scintillator, e.g., CsI, with similarbrightness, the CsI single crystal degrades by 30% following irradiationwith 1 krad.

In some approaches, scintillator material may include inorganic oxidescintillator materials. In one approach, oxide scintillator materialshave typically been labeled as emitting blue light and thus not bewell-matched for photodetectors. In one approach, a peak wavelength ofemission may be in the green-yellow light which is typically bettersuited for photovoltaic conversion. Conventional semiconductors forphotovoltaics have bandgap energies below the energy of blue light andare therefore inefficient at converting the light into current. Greenluminescence is advantageous for detection and conversion of energyallowing for the use of a more efficient silicon converter. Inparticular, zinc sulfide-type phosphors are generally understood to bedurable for long periods of time during cathode ray excitation (e.g.,black and white televisions). However, oxide scintillators tend to besingle crystal and, thus, are limited by geometry of the scintillator.

As illustrated in FIG. 2, in a preferred approach, a device includesthin layers 207 of a scintillator material 202 having a high Zscintillation property (e.g., a high effective atomic number) forproviding an effective radiation shield (e.g., high stopping power) andfor attenuating x-rays. Thus, in preferred approaches, a minimalthickness th_(Sc) of scintillation material 202 separates the betaemitter material 204 and photovoltaic portion 214.

In some approaches, the thickness th_(Sc) of the scintillator materialmay be determined by several properties of the device. For instance, atleast one of the following properties may be considered for determiningthe thickness th_(Sc) of the scintillator material: radiation harness ofthe photovoltaic material such that damage to the photovoltaic by thebeta emissions is minimized, a total amount of beta emitter materialincluded in the device, the effective atomic number (Z) and physicaldensity of the scintillator material for optimal power output, etc.

In one approach, an optimal thickness th_(Sc) of the scintillatormaterial may be based on power density of the battery, and the densityand atomic number of the scintillator material. For example, a thicknessof the scintillator material may be relative to the range of the betaemission particle passing through the scintillator material, such that,1 MeV beta emission that has a range of 200 μm, then a thickness of thescintillator may preferably be 200 μm to fully attenuate the energy ofthe beta emission particle. These values are by way of example only andnot meant to be limiting in any way.

In one approach, the thickness th_(Sc) of the layer 207 of scintillatormaterial 202 may be in a range of about 150 μm to up to about 3 mm (3000μm). In some approaches, the thickness th_(Sc) of the layer 207 ofscintillator material 202 may be tuned according to the radioisotope ofthe system. For example, in a system including Tl-204, the lower rangeof thickness may be appropriate, e.g., 150 μm to about 500 μm, whereasfor a system including Sr-90, the upper range of thickness may beappropriate, e.g., within the mm range.

The photovoltaic portion 214 may be a single power producing unit thatincludes a layer 210 of photovoltaic material 208 above the layer 205 ofbeta emitter material 204 with the layer 207 of scintillator material202 therebetween.

According to one embodiment, the photovoltaic portion 214 converts thephotons 209 generated by the scintillator material 202 to electricitye−, e.g., electrical current. In any approach, the photovoltaic portionmay be comprised of materials known in the art. In preferred approaches,the photovoltaic layer has an absorption spectrum well-matched to theemission spectrum of the scintillator material. The energy of theemission of the scintillator material is preferably equal to or greaterthan the band gap of the photovoltaic material to ensure the higherefficiency with the band gap light. For example, the energy needed tocreate electricity flow is preferably higher than, but nearly exactly,the band gap of the photovoltaic material and thus a wide band gap forthe photovoltaic material may likely reduce power generation. The bandgap of the photovoltaic material is preferably lower than, but nearlyequal to, the energy of the photons that are emitted from thescintillator material.

In one approach, a photovoltaic layer includes a III-V semiconductorthin film photovoltaic material. For example, the photovoltaic portionof a preferred device includes a III-IV semiconductor material, such asIndium gallium phosphide (InGaP), that demonstrates higher radiationhardness and thus tolerates higher radiation dose compared to thematerial used in conventional photovoltaics.

In some approaches, the photovoltaic material includes a standardwide-band gap semiconductor material. For blue emission, photovoltaicmaterial may include silicon carbide, gallium nitride, etc. In anexemplary approach, indium gallium phosphide (InGaP) provides a singlejunction III-V that matches a desired spectrum, e.g., approximately 1.9eV bandgap. It is not preferable to have multiple junctions that limitcurrent and decrease power. In contrast a multi-terminal junction maynot limit current or decrease power.

Moreover, in some approaches, the photovoltaic layer includes aradiation hard photovoltaic material. Preferably, an exemplaryphotovoltaic material balances optimal brightness with radiationhardness. In one approach, a radiation hard photovoltaic material ispreferred to minimize the distance between the radioisotope source(e.g., beta emitter material) and the photovoltaic portion. In oneapproach, a thickness th_(PV) of the photovoltaic portion 214 may be ina range of about 1 μm to about 500 μm, preferably in a range of about 5μm to about 20 μm. The range of thickness may be limited on the lowerend by ability to produce a functioning photovoltaic, e.g., about 1 to 5microns. The range of thickness may be limited on the upper end due toconstraints on power density. A very thick (100's of microns)photovoltaic may likely reduce power density by occupying more volumewithout contributing more power.

In a preferred embodiment, each layer in the stack 206 of layers of theproduct 200 (e.g., a photovoltaic cell) has an optimal minimal thicknessfor maximum efficiency and power density. In one approach, a thinnerlayer of the photovoltaic layer 210, approximately 5 to 10 μm, may allowfor higher power density.

In one approach, the photovoltaic material may include amorphoussilicon. Amorphous material tends to have greater radiation hardnessthan the single or polycrystalline analog, however the electricalproperties of amorphous silicon which influence charge collectionefficiency tend to be inferior to the single crystal material, in termsof the minority carrier diffusion and minority carrier lifetime. Otherphotovoltaic materials may include: CIGS, GaP, CdTe, CdSe, black-Si,GaAs, CdS, ZnTe, AlP, InP, etc.

As illustrated in the schematic drawing of a photovoltaic device 300 inFIG. 3A, a thin layer 302 of a III-IV semiconductor material, e.g.,InGaP, may be positioned adjacent a metal layer 304. In some approaches,the thickness th_(ML) of the metal layer 304 may be greater than thethickness th_(III-IV) of the thin layer 302 of III-IV semiconductormaterial. In preferred approaches, a ratio of the thickness th_(III-IV)of the thin layer 302 to the thickness th_(ML) of the metal layer 304may be about 1:5. For example, a thickness th_(III-IV) of the thin layer302 of III-IV semiconductor (e.g., InGaP) may be about 1 μm and thethickness th_(ML) of the metal layer 304 may be about 5 μm. Preferably,the metal layer 304 provides mechanical support as a backsidemetallization to the semiconductor material of the photovoltaic. Inpreferred approach, a minimum thickness of the photovoltaic device maybe limited by the metal layer that functions as backside metallization.A minimum thickness of the metal layer 304 may be approximately 5 μm. Insome cases, a backside metal contact having a thickness of less than 5μm may result in insufficient mechanical support to maintain thestructural integrity of the semiconductor layer, e.g., the semiconductormay crumble.

FIG. 3B is an image of a thin film photovoltaic device where thesemiconductor layer has a thicker backside metal contact providingmechanical support.

Referring back to FIG. 2, according to one embodiment, a reflectivelayer 203 may be positioned on the surface of the layer 205 of betaemitter material 204 between the beta emitter material 204 and thescintillator material 202. A reflective layer may improve lightcollection. Moreover, a reflective layer may circumvent a parasiticabsorption of light on the radioisotope surface.

In preferred approaches, a thickness of the reflective layer 203 may bein a range of greater than about 1 nm to less than about 2 μm,preferably approximately 50 nm or less. A minimum thickness may bedependent in part on the wavelength of the light of the system and thecomposition of the reflective layer. For example, for visible light, aminimum thickness of 5 nanometers (nm) may be sufficient for a singlereflective layer comprising metal, and alternatively, a minimumthickness of approximately 1 μm may be sufficient for a dielectric(non-metallic) reflective layer, e.g., TiO₂.

A known reflective material may be used for the reflective layer 203,preferably a reflective material that is radiation hard. In oneapproach, the reflective layer comprises aluminum. In one approach, thereflective layer includes unoxidized aluminum.

A device 220 may include a plurality of stacks 206 of scintillator/betaemitter/photovoltaic layers, where each stack is positioned in a seriesin a vertical direction. The photovoltaic portion 214 may include layers210 a, 210 b of photovoltaic material 208 sandwiching the layer 205 a ofbeta emitter material 204 and layer 207 a of scintillator material 202therebetween. In various embodiments, the layers as described herein maybe configured and tuned depending on specific applications.

FIGS. 4A-4E depict products 400, 420, 440, 460, 480 in accordance withvarious embodiments. As an option, the present products 400, 420, 440,460, 480 may be implemented using and/or in conjunction with featuresfrom any other embodiment listed herein, such as those described withreference to the other FIGS, especially FIG. 2. Of course, however, suchproducts 400, 420, 440, 460, 480 and others presented herein may be usedin various applications and/or in permutations which may or may not bespecifically described in the illustrative embodiments listed herein.Further, the products 400, 420, 440, 460, 480 presented herein may beused in any desired environment.

FIG. 4A depicts a schematic drawing of a side-view of a 2D product 400,according to one embodiment. A single stack 406 may have a layer 405including a beta emitter material 404. In one approach, the beta emittermaterial may include a Tl metal. A reflective layer 403 may bepositioned above the beta emitter material 404. The reflective layer 403may include one or more of the following materials: aluminum, gold, etc.The product 400 preferably demonstrates minimal energy loss of the betaemission through the reflective layer.

A layer 407 of a transparent scintillator material 402 may be positionedabove the reflective layer 403. A photovoltaic portion 414 includes alayer 410 of photovoltaic material 408. The photovoltaic portion 414 ispositioned above the scintillator material 402. A beta emission β⁻ fromthe layer 403 of beta emitter material 404 passes through the reflectivelayer 403 to the scintillator material 402. The transparent scintillatormaterial 402 is configured to transform the beta emission β⁻ from thebeta emitter material 404 to an emitted photon (e.g., luminescence). Theemitted photon is subsequently collected in the layer 410 havingphotovoltaic material 408 configured to convert light emitted by thescintillator material to electricity.

In one approach, the thickness th_(Sc) of the layer 407 of scintillatormaterial is sufficient to protect the photovoltaic portion 414 from morethan negligible radiation damage from the layer 403 of beta emittermaterial 404. Negligible damage of the photovoltaic portion 414 isdefined as degradation of electrical output by no greater than 10% overthe service life of the product 400. In one approach, the thickness ofthe layer of scintillator material is sufficient to protect thephotovoltaic portion from significant radiation damage. Significantradiation damage is defined as a more than 10% reduction in theelectrical power output of the photovoltaic due to decreased opencircuit voltage or short circuit current caused by defects generated inthe photovoltaic cell by ionizing radiation.

In one approach, the layer 407 of scintillator material 402 may includesub-layers of scintillator material having the same or differentcompositions. Accordingly, the term “scintillator material” as usedherein to describe material at various locations in the product isinterpreted as allowing the use of the same or different compositions ofscintillator materials at different locations of the product.

In one approach, the product 400 may include a transparent shieldinglayer between the layer 410 of photovoltaic material 408 and the layer405 of beta emitter material 404. A thickness of the shielding layer maybe sufficient to protect the photovoltaic portion 414 from more thannegligible damage. The shielding layer may be formed of a scintillatormaterial 402 that is the same or different composition as thescintillator material 402 adjacent the beta emitter material 404. Inother approaches, the shielding layer may be formed of any knownradiation shielding material.

In various embodiments, the structure of the scintillator material, betaemitter material, and photovoltaic portion may be scaled from a minimumsize determined by the fundamental cell thickness to a larger size thatmay be only limited by the techniques for forming the structure. In oneapproach, as illustrated in FIG. 4A, a thickness th_(P) of a product400, comprised of scintillator material 402, beta emitter material 404,and a photovoltaic portion 414, may be in a range of approximately 150μm to greater than 400 μm. Further, miniaturization of the structure maybe limited by the thickness of scintillator included to shield radiationfrom the radioisotope to the photovoltaic. In an exemplary approach, aminimum thickness of the structure may be approximately 175 μm.

According to one embodiment, at least some of the beta emitter materialmay be intermixed with the scintillator material. As shown in FIG. 4B,in one approach, a product 420 may have a layer 425 of beta emittermaterial 424 sandwiched between layers of the transparent scintillatormaterial 422. The beta emitter material 424 may be as describedelsewhere herein. In one approach the beta emitter material 424 is mixedwith a transparent scintillator material in layer 425, as discussed inmore detail below.

Layers of photovoltaic material 428 are positioned below and above thescintillator material 422 opposite the beta emitter material 424. Theproduct 420 may include a single stack 426 of layers including layers421, 430 comprising a photovoltaic material 428, layers 423, 427comprising transparent scintillator material 422, and a layer 425 ofbeta emitter material 424 positioned between the layers 423, 427 oftransparent scintillator material 422. In other approaches, the product420 may include repeating structures of the stack 426 or portionthereof, one atop the other.

The photovoltaic portion may be configured as a single power producingunit. As shown in product 420 of FIG. 4B, the photovoltaic portion 434may include two layers 421, 430 of photovoltaic material 428 sandwichingthe beta emitter material 424 and scintillator material 422therebetween. In one approach, the layers 430, 421 of photovoltaicmaterial 428 may include layers and/or sub-layers of the same ordifferent compositions of photovoltaic material. The various layers ofphotovoltaic material comprise the single power producing unit of thephotovoltaic portion for producing electricity.

In one approach, the thickness th_(Sc) of each layer 423, 427 ofscintillator material 422 may be sufficient to protect the photovoltaicportion 434 from more than negligible radiation damage as describedherein. The thickness th_(Sc) of each layer 423, 427 may be different orthe same.

In one approach, a layer 425 of beta emitter material 424 may be inintimate contact with the scintillator material 422. In anotherapproach, one or more reflective layers are interposed between layer 425and the scintillator material 422.

In one approach, the layer 425 may include beta emitter material 424intermixed with a scintillator material. The scintillator material inboth the mixture in layer 425 and layers 423, 427 may have the same ordifferent compositions, in various approaches. Likewise, thescintillator material 422 in each layer 423, 427 may have the same ordifferent compositions as one another. The layers including scintillatormaterial may have sub-layers having different or the same compositionsof scintillator material.

In some approaches, the beta emitter material and sections ofscintillator material, e.g., the same or different compositions as thelayers of scintillator material, may be present in alternating portionsin the space between the layers of scintillator material. For example, aproduct 420 may have more than one series alternating layers of betaemitter material 424 and scintillator material 422 positioned betweenlayers 423, 427 of scintillator material 422 adjacent the photovoltaicportion 434.

In one approach, the product 420 may include transparent shieldinglayers (not shown) between the layers 421, 430 of photovoltaic material428 and the layer 425 of beta emitter material 424. A thickness of theshielding layer may be sufficient to protect the photovoltaic portion434 from more than negligible damage. The shielding layer may be formedof a scintillator material 422 that is the same or different compositionas the scintillator material 422 adjacent the beta emitter material 424.In other approaches, the shielding layer may be formed of any knowntransparent radiation shielding material.

In one embodiment, a product may include at least some of the betaemitter material homogenously intermixed with the scintillator materialthroughout the bulk of the scintillator material. The source of betaemissions may be a homogeneously distributed source in a layer ofscintillator material. As illustrated in FIG. 4C, a product 440 mayinclude a photovoltaic portion 454 positioned adjacent transparentshielding layers 443, 447 of any type described herein opposite a layer445 of homogenously intermixed material 444 of beta emitter material andscintillator material. For example, in a 2D product, the product 440 mayinclude a single stack 446 of layers including layers 441, 450comprising a photovoltaic material 448, shielding layers 443, 447 e.g.,comprising transparent scintillator material or other shieldingmaterial, and a layer 445 of a homogenously intermixed material 444 ofbeta emitter material and scintillator material positioned between theshielding layers 443, 447.

As shown in product 440 of FIG. 4C, the photovoltaic portion 454 mayinclude two layers 441, 450 of photovoltaic material 448 sandwiching thehomogenously intermixed material 444 of beta emitter material andshielding layers 443, 447 therebetween. In one approach, the layers 441,450 of photovoltaic material 448 may include layers and/or sub-layers ofthe same or different compositions of photovoltaic material. In oneapproach, the scintillator material of layer 445 may be present in theshielding layers 443, 447 between the layers 441, 450 of photovoltaicmaterial 448 and the layer 445 of homogenously intermixed material 444of beta emitter material and scintillator material.

In some approaches, a product may have one layer 450 of photovoltaicmaterial 448 and one shielding layer 447 positioned above the layer 445of homogenously intermixed material 444 of beta emitter material andscintillator material.

In one approach, the thickness th_(Sc) of each shielding layer 443, 447is sufficient to protect the photovoltaic portion 454 from more thannegligible radiation damage as described herein. The thickness th_(Sc)of each layer 443, 447 may be different or the same.

In one approach, a layer 445 of homogenously intermixed material 444 mayinclude beta emitter material in intimate contact with the scintillatormaterial.

In one approach, the layer 445 may include beta emitter materialintermixed with scintillator material between shielding layers 443, 447that comprise scintillator material. The scintillator material in boththe mixture and shielding layers 443, 447 may have the same or differentcompositions, in various approaches. Likewise, the shielding material ineach shielding layer 443, 447 may have the same or differentcompositions relative to one another. The various layers 443, 445, 447may have sub-layers having different or the same compositions ofscintillator material.

In some approaches, the layer 445 of homogenously intermixed material444 of beta emitter material and scintillator material and shieldinglayers/sections may be present in alternating portions in the spacebetween the layers of scintillator material. For example, a product 440may have more than one series of alternating layers of a 445 ofhomogenously intermixed material 444 and layers of scintillator materialin a space between shielding layers 443, 447 adjacent the photovoltaicportion 454.

Preferably, a thickness of each shielding layer 443, 447 is sufficientto protect the photovoltaic portion 454 from more than negligible damageas described herein. Again, each shielding layer 443, 447 may be formedof a scintillator material that is the same or different composition asthe scintillator material in the homogenously intermixed material 444 ofbeta emitter material and scintillator material. In other approaches,the shielding layer may be formed of any known radiation shieldingmaterial.

In one embodiment, at least one of the layers of scintillator materialhas a nonplanar surface that defines pockets in which the beta emittermaterial may be deposited. For example, as illustrated in the schematicdrawing of product 460 in FIG. 4D, a layer 463 of scintillator material462 has a non-planar surface 472 that defines pockets 465 in which thebeta emitter material 464 may be deposited. The product may be athree-dimensional (3D) structure such that the layer 463 of scintillatormaterial 462 is characterized by features 476 such as pillars, ridges,channels, etc. In one approach, the scintillator material may formridges such that the ridges form a cavity between adjacent ridges. Thecavity between the ridges of scintillator material may be filled withbeta emitter material. In preferred approaches, the surface area ismaximized between the beta emitter material and the scintillatormaterial.

In one approach, the beta emitter material 464 may be in contact withmore than one non-planar surface 472 of the layer 463 of scintillatormaterial 462. In one approach, the deposited beta emitter material 464may fill greater than 25% of the pocket 465 defined by the non-planarsurface 472 of the layer 463 of scintillator material 462. In anotherapproach, the deposited beta emitter material 464 may fill the pocket465 defined by the non-planar surface 472 of the layer 463 ofscintillator material 462 in a range of about 25% to about 100%.

In one approach, the product 460 includes a second layer 467 ofscintillator material 462 above the deposited beta emitter material 464.The photovoltaic portion 474 may include two layers 461, 470 ofphotovoltaic material 468 sandwiching the beta emitter material 464 andscintillator material 462 therebetween. In some approaches, a singlestack 466 of layers includes one layer 470 of photovoltaic material 468and one layer 467 of scintillator material 462 above the beta emittermaterial 464.

The thickness th_(Sc) of each layer 463, 467 of scintillator material462 may be sufficient to protect the photovoltaic portion 474 from morethan negligible radiation damage as described herein. The thicknessth_(ScNP) of the scintillator material having a non-planar surface maybe defined as the distance from the closest edge of the beta emittermaterial 464 to the closest edge of the photovoltaic portion 474. Thethickness th_(ScNP), th_(Sc) of each layer 463, 467 may be different orthe same.

For some embodiments, the size of the scintillator and beta emittermaterial structure may be measured in terms of volume. For example, asize of the 3D structure of the product 460, as illustrated in FIG. 4D,may be in a range of greater than about 0.2 cm³ to a size limited by thethickness of the radiation shielding between the beta emitter materialand photovoltaic portion. In one exemplary approach, the cumulative sizeof the scintillator, beta emitter material, and photovoltaic structuremay be approximately 5 cm³ to 40 cm³ but may be larger.

In one approach, a layer of the beta emitter material may be in intimatecontact with the scintillator material. As shown in a 3D product 460 inFIG. 4D, beta emitter material 464 deposited in the pockets 465 may bein intimate contact with the scintillator material 462.

In one approach, the beta emitter material 464 may be intermixed withscintillator material 462 between the layers 463, 467 of scintillatormaterial 462. The scintillator material in both the mixture and layersmay have the same or different compositions, in various approaches.Likewise, the scintillator material 462 in each layer 463, 467 may havethe same or different compositions. The layers including scintillatormaterial may have sub-layers having different or the same compositionsof scintillator material.

In some approaches, the beta emitter material 464 and layers/sections ofscintillator material 462, e.g., the same or different compositions asthe layers of scintillator material, may be present in alternatingportions in the space between the layers of scintillator material. Forexample, a product 460 may have more than one series of a 463 ofscintillator material 462 having a non-planar surface 472 that definespockets in which the beta emitter material 464 is deposited in a spacebetween layers 463, 467 of scintillator material 462 adjacent thephotovoltaic portion 474. For example, in a product having machinedscintillator material and deposition of beta emitter material, a producthaving a width greater than 20 to 50 μm, the aspect ratio of the depthto width may be less than approximately 5-10.

As noted above, in some approaches, the thickness of the scintillatormaterial 462 above and below the beta emitter material 464 is sufficientto protect the photovoltaic portion 474 from more than negligible damageas described herein. In one approach, the product 460 may includetransparent shielding layers (not shown) between the layers 461, 470 ofphotovoltaic material 468 and the deposited beta emitter material 464and scintillator material 462. A thickness of the shielding layer may besufficient to protect the photovoltaic portion 474 from more thannegligible damage as described herein. The shielding layer may be formedof a scintillator material 462 that is the same or different compositionas the scintillator material 462 adjacent deposited beta emittermaterial 464. In other approaches, the shielding layer may be formed ofany known radiation shielding material.

One or more reflective layers may be positioned between the beta emittermaterial 464 and the scintillator material 462.

According to another embodiment, as shown illustrated in the schematicdrawing of product 480 in FIG. 4E, a layer 496 of scintillator material482 has a non-planar surface 492 that defines pockets 485 in which thebeta emitter material 484 is deposited. The product may be a 3Dstructure such that the layer 496 of scintillator material 482 ischaracterized as a wall surrounding a pocket 485. The pocket 485 may bea cavity defined by non-planar surface 492 (e.g., walls) of the layer496 and possibly the layers 483, 487 of the scintillator material 482.

In one approach, the beta emitter material and scintillator material maybe arranged such that each layer of beta emitter is encapsulated by thescintillating material. In one approach, the beta emitter material 484may be in contact with more than one non-planar surface 492 of the layer496 of scintillator material 482 and/or the layers 483, 487 ofscintillator material 482. Preferably, the deposited beta emittermaterial 484 fills greater than 25% of the pocket 485 defined by thenon-planar surface 492 of the layer 496 of scintillator material 482. Inanother approach, the deposited beta emitter material 484 may fill thepocket 485 defined by the non-planar surface 492 of the layer 496 andlayers 483, 487 of scintillator material 482 in a range of about 25% toabout 100%.

In one approach, the product 480 includes a second layer 483, 487 ofscintillator material 482 above and/or below the deposited beta emittermaterial 484. The photovoltaic portion 494 may include two layers 481,490 of photovoltaic material 488 sandwiching the beta emitter material484 and scintillator material 482 therebetween. In some approaches, asingle stack 486 of layers includes one layer 490 of photovoltaicmaterial 488 and one layer 487 of scintillator material 482 above thebeta emitter material 484.

The thickness th_(Sc) of each layer 483, 487 of scintillator material482 may be sufficient to protect the photovoltaic portion 494 from morethan negligible radiation damage as described herein. The thicknessth_(Sc) of each layer 483, 487 may be different or the same.

In one approach, a layer of the beta emitter material may be in intimatecontact with the scintillator material. As shown in a 3D product 480 inFIG. 4E, beta emitter material 484 deposited in the pockets 485 may bein intimate contact with the scintillator material 482.

In one approach, the beta emitter material 484 may be intermixed withscintillator material 482 between the layers 496, 483, 487 ofscintillator material 482. The scintillator material in both the mixtureand layers may have the same or different compositions, in variousapproaches. Likewise, the scintillator material 482 in each layer 483,487 may have the same or different compositions. The layers includingscintillator material may have sub-layers having different or the samecompositions of scintillator material.

In some approaches, the beta emitter material 484 and layers/sections ofscintillator material 482, e.g., the same or different compositions asthe layers of scintillator material, may be present in alternatingportions in the space between the layers of scintillator material. Forexample, a product 480 may have more than one series of a 496 ofscintillator material 482 having a non-planar surface 492 that definespockets 485 in which the beta emitter material 484 is deposited in aspace between layers 483, 487 of scintillator material 482 adjacent thephotovoltaic portion 494. For example, in a product having machinedscintillator material and deposition of beta emitter material, a producthaving a width greater than 20 to 50 μm, the aspect ratio of the depthto width may be less than approximately 5-10.

As noted above, in some approaches, the thickness of the scintillatormaterial 462 above and below the beta emitter material 464 is sufficientto protect the photovoltaic portion 474 from more than negligible damageas described herein. In one approach, the product 480 may includetransparent shielding layers (not shown) between the layers 481, 490 ofphotovoltaic material 488 and the deposited beta emitter material 484and scintillator material 482. A thickness of the shielding layer may besufficient to protect the photovoltaic portion 494 from more thannegligible damage as described herein. The shielding layer may be formedof a scintillator material 482 that is the same or different compositionas the scintillator material 482 adjacent deposited beta emittermaterial 484. In other approaches, the shielding layer may be formed ofany known radiation shielding material.

One or more reflective layers may be positioned between the beta emittermaterial 484 and the scintillator material 482.

In early contemplated approaches of the present invention,configurations of the layers as described herein were found to provideless than optimal energy output. Several such approaches are describedbelow with reference to FIGS. 5A-5C. Though these approaches were foundto be less than optimal using current materials, they are inventive,nonetheless. None of the designs illustrated in FIGS. 5A-5C is drawn toscale.

An early design of a device 500 included a GYGAG scintillator material502, radioisotope source 504 (e.g., Tl metal), reflective layers 506,and a photodiode 508, as shown in the schematic drawing of FIG. 5A. Abeta emission β⁻ travels into the scintillator material 502 therebybeing converted to an emitted photon that travels to the photodiode 508.However, this device 500 has notable drawbacks. In particular, lightextraction can be challenging. Materials for the reflective layer 506,e.g., aluminum (Al), silver (Ag) and gold (Au), are not reflectiveenough for very high aspect ratio scintillation parts, for examplehaving an aspect ratio of 0.02×6×8. Self-absorption, evanescent waveproperty losses, etc. reduce percentage of light collected at thephotovoltaic. White diffuse reflector layers (e.g., reflectors) arepreferred (greater than 99% reflection) but typically have a thicknessthat is larger than desired. The cumulative thickness of a reflectivelayer results in a dramatically decreased power density. In someapproaches, a thickness of the reflective layer on the order of 1-5 μmmay be necessary for the full effect to be realized. A preferredthickness of the scintillator material is preferably at least 100 μm.Moreover, minimizing chord length (total path) of light in scintillatormaterial may likely improve collected efficiency.

In one approach, a stacked diode design, as illustrated in the schematicdrawings of FIGS. 5B and 5C, may improve light extraction but mayincrease radiation dose to diodes. FIG. 5B illustrates a stacked version520 of a device having a stacked diode 526 in which layers of GYGAGscintillator 522 sandwich layers of radioisotope 524, e.g., thallium(Tl) metal, therebetween. Additionally, a border layer 530 of GYGAGscintillator is vertically aligned along the horizontal diode stack 526of GYGAG/Tl layers, and a photovoltaic portion 528 is aligned on theopposite side of the GYGAG border layer 530 from the diode stack 526 ofGYGAG/Tl layers. An advantage of the stacked version 520 includes lowx-ray and beta emission dose to the photovoltaic, the GYGAG borderprovides protection of the photovoltaic. However, a drawback of thisapproach includes poor light extraction and thus lower than expectedefficiency.

According to another approach, FIG. 5C illustrates a second stackedversion 540 of a device including multiple sets of a GYGAGscintillator/Tl layer stack 546 comprised of two GYGAG scintillatorlayers 542 with a radioisotope layer 544, e.g., a Tl metal layer,positioned therebetween. The second stacked version 540 includes aphotovoltaic layer 548 positioned above and below each set of GYGAGscintillator/Tl layer stack 546. An advantage of the second stackedversion 540 is improved light collection from scintillation that in turnmay boost efficiency of power generation. A drawback of the secondstacked version 540 is an increased x-ray and beta emission dose to thephotovoltaic layers 548.

FIG. 6 depicts a flowchart of a method 600 for forming a product, inaccordance with one embodiment. As an option, the present method 600 maybe implemented to form materials such as those shown in the other FIGS.described herein. Of course, however, this method 600 and otherspresented herein may be used provide applications which may or may notbe related to the illustrative embodiments listed herein. Further, themethods presented herein may be carried out in any desired environment.Moreover, more, or less operations than those shown in FIG. 6 may beincluded in method 600, according to various embodiments. It should alsobe noted that any of the aforementioned features may be used in any ofthe embodiments described in accordance with the various methods.

According to one embodiment, a method 600 includes an operation 602 offorming a product that includes a transparent scintillator material, abeta emitter material having an energy of greater than 225 keV, and aphotovoltaic portion configured to convert light by the scintillatormaterial to electricity. The method 600 may be used to form any approachdescribed herein, including those described with reference to FIGS.2-5C.

In one approach, a product includes multiple layers of beta emittermaterial (e.g., radioisotope) coupled to a scintillating material madeof an optically transparent polycrystalline ceramic garnet-structurescintillator coupled to one or more photovoltaic portions. Thescintillating material may be fabricated using conventional techniquesthat would become apparent to one skilled in the art upon reading thepresent disclosure. For example, the scintillator material may be formedby pressing powdered ceramic nanoparticles, microparticles, etc. dopedwith an activator, e.g., cerium, to form a green body. The formed greenbody may be sintered in a controlled atmosphere to approximately 90%theoretical density. The sintered green body may be isostaticallypressed under high pressure to form an optically transparent ceramic atapproximately full density (100%). The transparent ceramic may then beannealed at high temperature to control the optical properties.

During the fabrication of the scintillator, a beta emitter material(e.g., radioisotope) may be placed in physical contact with thescintillator. In one approach, the geometry of the radioisotope may bein the form of micron-scale particles mixed in the scintillator duringpressing of the ceramic powder to form a homogenous source. In anotherapproach, the geometry of the beta emitter material may be as atwo-dimensional (2D) semiconductor structure (see FIGS. 4A-4C). In yetanother approach, the geometry of the beta emitter material may befilled in a three-dimensional (3D) semiconductor structure (see FIGS.4D-4E). The coupled beta emitter material and scintillator system may besurrounded by a thin (e.g., approximately 3 mm, in one approach) layerof transparent ceramic scintillator without any radioisotope in thescintillator material.

In one approach, the beta emitter material may include a mixture ofdifferent beta emitter materials, e.g., Sr-90 and Tl-204. In someapproaches, the different beta emitter materials may be combined in theapparatus. For example, a mixture of two or more beta emitter materialsmay be mixed prior to addition of the mixture as a layer of beta emittermaterial in the device. In another approach, the device may includemultiple single compositions of beta emitter material, e.g., a firstlayer is a first composition of beta emitter material, a second layerpositioned above the first layer is a second composition of beta emittermaterial that is different from the first, a third layer is the firstcomposition of beta emitter material, etc. Any combination of betaemitter material(s) may be included in the device and mixed prior toaddition or mixed in the device. These are examples only and are notmeant to be limiting in any way.

According to one embodiment, the scintillation layer (e.g., outerscintillation material layer) may serve to attenuate beta particles andreduce the flux of damaging x-rays that may otherwise irradiate thephotovoltaics on the surfaces of the scintillator structures. In variousapproaches, it is preferable for candidates for scintillators suitableto convert the radioisotope decay energy to photons to have effectiveradiation resistance. For example, and not meant to be limiting, somepreferred scintillators may demonstrate a radiation resistance togreater than 500 Mrad of electron irradiation.

In one approach, inorganic oxide scintillators having a luminositygreater than 30,000 photons/MeV, either single crystal orpolycrystalline ceramic, may be activated with rare earth elements. Invarious approaches, rare earth elements include GYGAG(Ce), YAG:Ce,LuSiO:Ce, LuYAlG:Pr, etc. in varying ratios of their constituents. Theseare by way of example only and are not meant to be limiting in any way.

The bandgap of the photovoltaic cells and the energy of the photonsemitted by the scintillator are preferably well matched. Possiblephotovoltaic cell materials include InGaP, SiC, GaN, amorphous Si,diamond, AN, GaAs, c-Si, AlGaN, AlGaP, hexagonal BN, CdTe, CIGS,inorganic perovskite materials, etc.

In one approach, on each face of the layer of beta emitter material(e.g., radioisotope source) a thin (˜50 nm) unoxidized aluminum coatingmay be applied to provide reflection of photons incident on theradioisotope layer to increase the light collection efficiency of thedevice.

In various approaches, the product may be formed by creating laminatedlayers using techniques as would become apparent to one skilled in theart reading the present disclosure. For example, a layer of photovoltaicmaterial may be fabricated in one process, and then the subsequentlayers of scintillator material, beta emitter material, scintillatormaterial, and a second layer of photovoltaic material are seriallylaminated above each other, e.g., as in a tape roll process.

Experiments

GYGAG(Ce) Scintillator Material

As shown in the image of FIG. 7A, a GYGAG(Ce) scintillator 700 is atransparent ceramic scintillator having a diameter of 16 mm.

Irradiation of GYGAG(Ce) Scintillator Material

FIG. 7B is a plot of β-irradiated GYGAG(Ce) emission and absorptionspectra of samples having varying thicknesses, 1 mm (□), 10 mm (●), and25.4 mm. The light yield of GYGAG(Ce) tends to be in a range of 20 to 25eV/photon. For an emission photon exhibiting about 2.2 eV, the incidentradiation-to-photon conversion efficiency of a GYGAG(Ce) material may bein a range of about 8 to 11% as shown in the spectral range of 450 nm to700 nm.

FIG. 8A shows the radiation-hardness of a GYGAG(Ce) scintillator inresponse to beta irradiation. The scintillation efficiency was measuredon each side of the scintillator (Side 1 and Side 2), to show the lightyield of a transparent scintillator before and after a high dose of betairradiation, 550 Mrad dose of 1 MeV electrons for 4 hours. The 550 Mraddose is approximately equivalent to about 5% of a 10-year dose and thusmay represent longevity of scintillator. There is no difference in thescintillation efficiency between the Side 1 and Side 2 of the irradiatedscintillator sample (within experimental error), nor any difference inthe scintillation efficiency before and after irradiation (all curveshave the same pattern).

Electron irradiations were also performed using electrons ranging inenergy from 500 keV to 2 MeV. The samples were held below 200° C. andirradiated to doses from 3 to 310 Grad. Within experimental error of5-10% there was no difference in scintillation efficiency or light yieldbetween the irradiated and unirradiated scintillator samples. This wasverified using alpha, beta, and gamma radioluminescence of each of theirradiated samples on both the irradiated and unirradiated surfaces andcompared to the pre-irradiated results.

Alpha particle irradiation of a GYGAG(Ce) scintillator has a differenteffect on Side 1 and Side 2 of an irradiated scintillator sample. FIG.8B shows the effect of alpha particle irradiation of a GYGAG(Ce)scintillator with an energy of 0.5 to 3.5 MeV (compared to the 1 MeV ofbeta irradiation of FIG. 8A in which the fluence of about 10¹⁶/cm² isthe same as for the beta irradiation test). Looking to the emissionmaxima, Side 1 (solid line) receiving the irradiation of alpha particlesdemonstrates an emission of just below approximately 1.6e5 whereas Side2 (□, the non-irradiated side) demonstrates a notable 98% degradation oflight yield, at approximately 3.6e3. For both irradiation tests (betaemission irradiation, FIG. 8A, and alpha particle irradiation, FIG. 8B)the transmission and absorption demonstrate less % error. Side 1irradiated with alpha particles produces 2.3% of the emission of thenon-irradiated side.

Uses

Various embodiments described herein may be used as and/or for use witha radioisotope battery, sensors, electronics, light sources, radiationhard electronics, nuclear fuel monitoring, spacecraft, and remoteelectronics.

The inventive concepts disclosed herein have been presented by way ofexample to illustrate the myriad features thereof in a plurality ofillustrative scenarios, embodiments, and/or implementations. It shouldbe appreciated that the concepts generally disclosed are to beconsidered as modular, and may be implemented in any combination,permutation, or synthesis thereof. In addition, any modification,alteration, or equivalent of the presently disclosed features,functions, and concepts that would be appreciated by a person havingordinary skill in the art upon reading the instant descriptions shouldalso be considered within the scope of this disclosure.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of an embodiment of the presentinvention should not be limited by any of the above-described exemplaryembodiments but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. A product, comprising: a transparent scintillatormaterial; a beta emitter material having an end-point energy of greaterthan 225 kiloelectron volts (keV); and a photovoltaic portion configuredto convert light emitted by the scintillator material to electricity,wherein the scintillator material is characterized as not exhibiting asignificant degradation of light output under continuous exposure toradiation energy at 1 megaelectron volt (MeV) to a dose of at least 1gigarad for a duration of one year.
 2. The product as recited in claim1, wherein the scintillator material is in the form of a ceramic.
 3. Theproduct as recited in claim 1, wherein the scintillator material is inthe form of a single crystal material.
 4. The product as recited inclaim 1, wherein the scintillator material exhibits a light output ofgreater than about 30,000 photons per megaelectron volt (MeV).
 5. Theproduct as recited in claim 1, wherein at least some of the scintillatormaterial includes an activator.
 6. The product as recited in claim 1,wherein the beta emitter material has an average radiation energy ofless than about 1 MeV.
 7. The product as recited in claim 1, wherein atleast some of the beta emitter material is intermixed with thescintillator material.
 8. The product as recited in claim 7, wherein theat least some of the beta emitter material is homogeneously intermixedwith the scintillator material.
 9. The product as recited in claim 1,wherein a layer of the beta emitter material is in intimate contact withthe scintillator material.
 10. The product as recited in claim 1,comprising a reflective layer positioned between the beta emittermaterial and the scintillator material.
 11. The product as recited inclaim 1, wherein a thickness of the photovoltaic portion is in a rangeof about 1 micron to about 500 microns.
 12. The product as recited inclaim 1, wherein the photovoltaic portion includes two layers ofphotovoltaic material sandwiching the beta emitter material andscintillator material therebetween.
 13. The product as recited in claim12, wherein the scintillator material is present in two layers betweenthe layers of photovoltaic material and the beta emitter material. 14.The product as recited in claim 13, wherein a thickness of each layer ofscintillator material is sufficient to protect the photovoltaic portionfrom significant radiation damage.
 15. The product as recited in claim13, wherein the beta emitter material is intermixed with thescintillator material between the layers of scintillator material. 16.The product as recited in claim 13, wherein at least one of the layersof scintillator material has a nonplanar surface.
 17. The product asrecited in claim 13, wherein the beta emitter material and sections ofthe scintillator material are present in alternating portions in thespace between the layers of scintillator material.
 18. The product asrecited in claim 12, comprising transparent shielding layers between thelayers of photovoltaic material and the beta emitter material, wherein athickness of each shielding layer is sufficient to protect thephotovoltaic portion from more than negligible radiation damage.
 19. Amethod, comprising: forming a product, the product comprising: atransparent scintillator material; a beta emitter material having anenergy of greater than 225 kiloelectron volts (keV); and a photovoltaicportion configured to convert light emitted by the scintillator materialto electricity, wherein the scintillator material is characterized asnot exhibiting a significant degradation of light output undercontinuous exposure to radiation energy at 1 megaelectron volt (MeV) toa dose of at least 1 gigarad for a duration of one year.
 20. The productas recited in claim 1, wherein the transparent scintillator material ispresent as a first layer, wherein the beta emitter material is presentas a second layer extending along the first layer.
 21. A product,comprising: a transparent scintillator material; a beta emitter materialhaving an end-point energy of greater than 225 kiloelectron volts (keV);and a photovoltaic portion configured to convert light emitted by thescintillator material to electricity, wherein a thickness of thephotovoltaic portion is in a range of about 1 micron to about 500microns.